The present invention relates to a focus control apparatus and a focus control method suitable for use in a video camera or the like.
A consumer video camera has employed an autofocus apparatus for automatically focusing a lens on an object.
It is well known that, in order to discriminate whether or not a lens is in focus or out of focus, it is sufficient to discriminate whether contrast of a video signal obtained by an image pickup is high or low. In other words, if the contrast is high, then the lens is in focus. If on the other hand the contrast is low, then the lens is out of focus. A high-frequency component is extracted from the video signal obtained by an image pickup, and a data obtained by integrating the high-frequency component in a predetermined set area is generated. It is possible to discriminate whether the contrast is high or low, by using the integrated data. The integrated data is indicative of how much there is the high-frequency component in the predetermined area. In general, this data is called an estimation value. Accordingly, it is possible to realize the autofocus method by driving a focus lens so that the estimation value should be maximum (i.e., the contrast should be maximum).
When an object, a background and an image pickup condition are not changed, a noise resulting from an external disturbance is seldom included in the estimation value. However, when a video camera for imaging a moving picture is used, the object, the background and the image pickup condition are changed on a real time base. As a result, the estimation value sometimes includes the noise. Therefore, it is very difficult to detect a precise estimation value from a picture of an object changed on a real time base.
Moreover, if the estimation value includes any noise, when the focus lens is passed by a focus position, the estimation value does not become maximum, and, conversely, when the focus lens is not located on the focus position, the estimation value becomes maximum. This may lead to misjudgment of the focus position.
As a result, in order to detect a lens position where the estimation value is maximum, the focus lens is reciprocated in the vicinity of the focus position where the estimation value should be maximum. Therefore, it takes a considerable time to focus the focus lens.
If luminance is low, the estimation value obtain in this case constantly includes the noise, which makes it difficult to detect the focus position. Therefore, it is impossible to carry out a high-accuracy focus control.
A sampling point used for obtaining the estimation value does not always coincide with the position where the estimation value becomes maximum. Moreover, since the focus lens is reciprocated plural times in the vicinity of this maximum estimation value position, i.e., the focus position to determine the focus position, it takes a considerable time to determine the focus position.
For example, it is sometimes observed that an image picked up by a high-end video camera apparatus for use in a broadcasting station or for professional use is transmitted on the air as a live relay broadcast. If it takes a considerable time to carry out the autofocus operation when such live relay broadcast is carried out, consequently a video signal indicative of a blurred picture is transmitted on air. Therefore, a simplified, inexpensive and small autofocus apparatus such as that used in a consumer video camera is not necessary for the video camera for use in the broadcasting station or for professional use, but a high-accuracy focus control and a high-speed focus control are required therefor.
It is an object of the present invention to provide a focus control apparatus and a focus control method which can control a focus at high speed with high accuracy.
A focus control apparatus according to the present invention is a focus control apparatus having an imaging means for imaging an object through a focus lens to output an electric signal corresponding to the object, including an extracting means for extracting a high-frequency component of the electric signal output from the imaging means, an estimation value generating means for generating an estimation value indicative of a focus state of the object based on the high-frequency component output from the extracting means, a storage means for storing the plurality of estimation values changed as the focus lens is moved in response to a focus lens position in order to obtain a just focus position, a selecting means for selecting a plurality of estimation values to be used for calculation of the just focus position from the estimation values stored in the storage means, and a control means for calculating the just focus position based on the plurality of estimation values selected by the selecting means and lens positions corresponding to the plurality of selected estimation values.
A focus control method according to the present invention is a focus control method of moving a focus lens of a video camera to a just focus position, including a) a step of extracting a high-frequency component of an electric signal output from an imaging means, b) a step of generating an estimation value indicative of a focus state of an object based on the high-frequency component extracted in the step a), c) a step of storing the plurality of estimation values changed as the focus lens is moved in response to a focus lens position, d) a step of selecting a plurality of estimation values to be used for calculation of the just focus position from the estimation values stored in the step c), e) a step of calculating the just focus position based on the plurality of estimation values selected in the step d) and lens positions corresponding to the plurality of selected estimation values, and f) a step of moving the focus lens to the just focus position.
According to the present invention, since the just focus position is calculated based on a plurality of selected estimation values and the lens positions corresponding to the plurality of selected estimation values, even if the estimation value includes a noise or the estimation value constantly includes a noise when the luminance is low, it is possible to carry out the focus control with high accuracy. Even if the focus lens is passed by the just focus position only once, then it is possible to calculate the just focus position. Therefore, it is possible to determine the just focus position at high speed to that extent.